The official answer is: you can't. (hint: I wasn't happy with that answer either)
Atlassian continuously deploys updates for Jira Cloud users, and (ignoring phased rollouts) all Cloud users are by definition on the latest version.
For most users, there really is no need to know. You're always up-to-date, and what's not to love about that? Read about the delivered updates (API users here) as needed to catch up to all the recently deployed goodness.
Since Atlassian Cloud product users cannot roll-back to a previous version, the only possible value I see is to be informed that a software update has occurred.
And along those lines, I've seen some use cases (e.g. external automated activities) that at least want to know when dependent software (Jira in this case) is changed, so that operations can be validated as still working (or broken) as soon as possible after the update.
For those that really have a need to know, I might have a solution to offer. Use the JIRA Rest API and query your cloud Jira instance for the "serverInfo". It's documented here. Note that anonymous access is permitted here, so no authentication is needed.
Here's an example of what is returned today in the response JSON:
This suggests that our instance was last updated between 1-2 days ago. My assumption is that the next buildNumber will be larger.
Not strictly true - if you regenerate software but end up with exactly the same product, the build could stay the same but the build date change.
It's another point that underlines why asking "what version is Cloud" is utterly pointless, the answer is of no possible use to you.
As @Mykenna Cepek says, you can't, but they're even more right when they lean into "it doesn't really matter". The two big points there are
(Don't get me wrong, there's a load of perfectly valid reasons for asking, but they're mostly rooted in "we control X". When you sign up for a service, you have chosen to abrogate that, you don't, and the version becomes almost entirely irrelevant)
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